In a Resource Usage view, set focus to the task assignment line just below any resource line (i.e. click on any field on the assignment line). Then hit Insert key to open a blank line and try to enter name of a task that does not exist in your schedule. Project will display the dialog box below if the checkbox Automatically add new … and tasks” has been cleared ([ ]):
Click “Yes” to create the new task assignment and add this task to your schedule or click “No” to cancel. What is the use of this feature ? I guess it is obvious; if that option is on ([x]), Project will silently add a new task to your schedule using the mistyped name.